Poly(ethylene terephthalate) (PET) has been modified with low levels of isophthalic acid (i.e., up to about 3 mole percent) primarily to adjust (decrease) the rate of crystallization of PET, in order to obtain amorphous articles such as partially oriented yams, amorphous sheets for thermoforming or biaxially oriented film fabrication, and bottle/jar preforms for injection stretch-blow molded containers. For most applications, however, it is most desired that the final fabricated article exhibit a high level of crystallinity, typically measured by density value. Crystallinity imparts high stiffness, low creep, improved barrier properties and other desired performance properties.
There is a need to modify PET to produce fabricated articles that have a combination of high density and enhanced performance properties, such as stiffness and barrier to gases. We have discovered that PET modified with a combination of isophthalate and 2,6-naphthalate moieties provides such improved polyesters. We have also discovered that PET modified with isophthalate moieties at levels at or greater than about 5 mole percent provides such improved properties.